The electric motors of various designs are available to meet the needs of various classes of industry. So the selection of electric motor itself has become an important and tedious process. The conditions under which an electric motor has to operate and the type of load it has to handle, determine its selection.
The various factors that are to be considered in the selection of an electric motor for a particular service are: 1. Nature of Electric Supply 2. Types of Drives 3. Nature of Load 4. Electrical Characteristics 5. Mechanical Considerations 6. Service Capacity and Rating 7. Appearance 8. Cost Considerations.
A large number of factors are to be considered in making the choice of an electric motor for a given drive. Although in the list of various considerations the cost has been placed at the end, yet the final choice has to be governed by it. The motor selected must fulfill all the necessary load requirements and at the same time it should not be very costly if it has to be a commercial success.
In any particular case some of the factors may be definitely fixed by existing circumstances while others will probably be conflicting and, therefore, for making most satisfactory choice understanding of various possibilities is necessary. The basic problem is of matching the mechanical output of the electric motor with the load requirements, i.e.,’ to provide a motor with the correct speed-torque characteristics as demanded by the load.
1. Nature of Electric Supply:
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The electric supply available may be 3-phase ac, single-phase ac or dc.
In case three-phase ac supply is available, polyphase induction motor, squirrel cage type for small ratings and slip-ring type for higher ratings may be used, provided this suits the requirements of the load. In the cases where speed variation is required these cannot be conveniently used, so pole changing motors or motors with stepped pulleys may be used. Where accurate control of speed is required, Schrage motors may be used. Use of single-phase motors is limited to small loads only owing to their limited outputs.
DC motors are not used so widely as ac ones.
There are several reasons for this, the most important of which are given below:
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1. At present a large percentage (in fact, the whole) of the electrical energy used for domestic and commercial purposes is generated in ac form because of economic and technical reasons, i.e., supply available is usually ac. Additional equipment is, therefore, required for converting existing ac supply into dc supply.
2. DC motors have commutators that are subject to trouble resulting from sparking, brush wear, arc-over and the presence of moisture and destructive fumes in the surrounding air.
3. DC motors are generally more expensive than ac motors for similar working conditions.
On contrary, ac motors have the advantages of possessing few working parts, requiring less maintenance and replacement of spares and providing uninterrupted long service life. Three- phase induction motors have high efficiency and provide high starting and uniform operating torques and balanced loading of supply.
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DC motors are nevertheless used in a large number of applications, especially when their excellent torque and speed operating characteristics cannot be duplicated by ac motors. Moreover, the speed control that can be provided for dc motors is far more flexible and satisfactory than that which can be provided for ac motors. These requirements are particularly significant in connection with services such as operation of traction equipment, hoists and elevators. In some cases, such as in electric excavators, steel mills and cranes, the speed control is so important that existing ac supply is converted into dc supply in order to employ dc motors.
With electrical utilities transmitting energy on ac, the usual arrangement used to be a motor-generator set (a dc generator driven by an ac motor) supplying power to a dc motor. However, the advent of solid state diodes and silicon controlled rectifiers (SCRs) eliminated the need for the motor- generator set and made dc motor drives more versatile and more popular than even before.
Nature of dc supply available, to some extent affects the performance, and therefore, size of the motor required. DC supply available from a dc generator will be more or less smooth but the output voltage from thyristor converter consists of a dc component and ac harmonic components. Torque or mechanical power is developed by the dc component of the current whereas armature heating is developed by the effective or rms value of current.
The form factor for half-wave three- phase thyristors may be taken as 1.2 while for full-wave three-phase thyristors it is 1.1. This increases electrical losses, and, therefore, heating is 5 to 7% more for three-phase full- bridge converters while for three-phase half-bridge convertors it is from 15 to 20%. Thus when a dc motor is driven from thyristor convertor, slightly large size motor will be required for a given kW power requirement.
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The commercial ac power is generated at a frequency of 50 Hz but this frequency is not capable of meeting the requirement of certain industries, defence services and railway signaling. As such, there has been recent trend towards development of high frequency power generation.
Relative merits and demerits of high-frequency and power-frequency equipment are enumerated below:
Merits of High Frequency:
1. The output of the equipment increases with the increase in frequency or speed, as obvious from the following equations-
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The output of an alternator per phase,
P = 4KfKp Kd ɸfT × I cos ɸ …(1.1)
where Kf, Kp and Kd are the form factor, pitch factor and distribution factor, ɸ is the useful flux per pole in Webbers, f is the frequency in hertz, T is the number of turns in series per phase, I is the output current per phase and cos ɸ is the power factor.
The output of a motor is given as-
P = KD2LN …(1.2)
where D is diameter, L is the length and N is the speed of motor in rpm.
For a given power output, high frequency machine may be of one-eighth of the weight of the 50 Hz machine. Because of high power-weight ratio, relative cost of the equipment is reduced with the increase in frequency. Reduction in weight of equipment makes also transportation and handling of equipment easy and convenient at high altitude for defence services.
2. For the generation of high frequency power, along with the small sized generator, the prime mover will also be of smaller size. This will result in reduction of high noise problem.
3. High frequency equipment requires less maintenance and has low operating cost, higher efficiency and long life span requiring minimum repairing cost.
4. More reliable service is provided, in arduous conditions of high altitude, low temperature and high humidity, by high frequency equipment.
The drawback of high frequency equipment is that it has a problem of increased power losses due to hysteresis and eddy currents. However, this problem of large power losses can be overcome by using low loss materials, proper flux density and adequate cooling arrangements.
2. Types of Drives:
The various types of electric drives used in industry may be divided into three types: the group, individual and multi- motor drives.
i. Group Drive:
By group drive is meant a drive in which a single electric motor drives a line shaft by means of which an entire group of working machines may be operated. It is also sometimes called the line shaft drive. The line shaft is fitted with multi-stepped pulleys and belts that connect these pulleys and the shafts of the driven machines and serve to vary their speed. This drive is economical in consideration of the first cost of the motors and control gear.
A single motor of large capacity costs less than the total cost of a number of small motors of the same total capacity viz., a single 100 kW motor costs much less than that of ten motors of 10 kW each. Since all the machines may not be used on full load at the same time, therefore, the kW rating of the motor for group drive can often be less than the aggregate of kW output ratings of all the individual motors, and further causes reduction in cost.
The efficiency and power factor of a large group drive motor will be higher, provided it is operated fairly near its rated load. If the machines are liable to short but sharp overloads, group drive is again advantageous, because 100% overload on an individual machine will cause hardly 10% overload when being driven by group drive.
Owing to its following drawbacks and objectionable features, this form of drive has become obsolete nowadays and the modem trend is to employ individual and multimotor drives:
a. In group drive, speed control of individual machine is very cumbersome using stepped pulleys, belts etc.
b. Owing to use of line shafting, pulleys and belts group drive does not provide good appearance and is also less safe to operate.
b. In group drive since machines have to be installed to suit the layout of the line shafting, as such flexibility of layout of the various machines is lost. Also, it is not possible to install any machine at a distant place.
d. The possibility of installation of additional machines in an existing industry is limited.
e. If at a certain instance all the machines are not in operation, the motor will operate at low capacity and, therefore, operation efficiency will be low.
f. The breakdown of large single motor causes all the operations to be stopped. In some processes, where stoppage of one operation causes stoppage of whole sequence such as in textile industry or flour mills, this drawback is not effective and group drive, therefore, can be used.
g. Considerable power loss takes place in the energy transmitting mechanism.
h. The level of the noise produced at the work site is quite large.
Group drive is adopted, when existing factories are changed over from engine drive to electric motor drive simply by replacing the oil or steam engine by an electric motor of corresponding output retaining all the old shafts and belts.
ii. Individual Drive:
In an individual drive, a single electric motor is used to drive one individual machine. Example of such a drive are single-spindle drilling machines, various types of electrical hand tools and simple types of metal working machine tools and mechanisms. Though it costs more than group drive, but each operator has complete control of his machine, which enables him to vary its speed if necessary and stop while not is use, thus eliminating no-load losses.
The machines can be placed in any desired position and can be moved very easily. The motor and its control unit can be built as an integral part of the machine which results in good appearance, cleanliness and safety. For driving heavy machines such as for lifts, cranes, shapers, lathes etc. and for the purposes where constancy of speed and flexibility of control is required, such as in paper mills and textile industry, individual drive is essential.
For new factories individual drive is preferred, as it causes some saving in the cost of superstructure because of being much lighter and less expensive.
In case of individual drive too, the energy is transmitted to the different parts of the same mechanism by means of mechanical parts (such as gears, pulleys etc.). Thus some power loss occurs in the energy transmission mechanism. This drawback is overcome in the case of multimotor drives.
iii. Multimotor Drive:
It consists of several individual drives each of which serves to operate one of many working members or mechanisms in some production unit. For example, in travelling cranes, there are three motors one for hoisting, another for long travel motion and the third for cross travel motors. Such drive is essential in complicated metal-cutting machine tools, paper making machines, rolling mills, rotary printing machines and similar types of other machinery. The use of multimotor drive is continuously expanding in modern industry as their advantages outweigh the increase in capital cost as compared to the group drives.
The use of individual and multimotor drives has enabled introduction of automation in production processes, which in turn has considerably increased the productivity of various industrial organisations. Complete or partial automation helps to operate various mechanisms at optimum conditions and to increase reliability and safety of operations.
3. Types of Loads:
There are two types of loads—those which provide active torques and those which provide passive torques. Active torques are due to either gravitational force (as in case of hoists, lifts or elevators and railway locomotive operating on gradients) or deformation in elastic bodies. Such torques are also developed during compression or release of springs. Since functioning of hoisting mechanism, operation of locomotives on gradients and compression/release of springs are all associated with a change in potential energy of the drive, active torques are also closely connected to the potential energy.
In upward movement of load/compression of a spring, the stored potential energy increases and the active torque developed opposes the action that takes place i.e., the torque is acted against the upward movement/compression. On the other hand in the downward movement of load/release of a spring the stored potential energy decreases and torque associated with it helps the action. Thus, it can be seen that the active torque continues to act in the same direction even after the direction of the drive has been reversed.
Passive torques are those due to friction or due to shear and deformation in inelastic bodies (lathes, pumps, fans etc.). They always oppose the motion, retarding the rotation of the driven machine. Moreover, with change in direction of motion, the sense of torque also changes. For example, when a weight is raised upward, the friction torque adds to the useful torque but when lowered down it subtracts from the latter.
4. Electrical Characteristics:
(a) Operating or Running Characteristics of Electric Motors:
While studying electrical behaviour of a machine under normal operating conditions, the speed-torque characteristic, speed- current characteristic, losses, efficiency, magnetising current and power factor at various loads are kept in view. The last two factors, i.e., magnetising current and power factor are to be considered in case of ac motors only.
Other features, such as temperature rise and insulation strength, are taken care of by Indian Standard Specifications, therefore if a motor is purchased built by a reputed manufacturer in accordance with the appropriate specifications, it will prove satisfactory in these respects.
DC Motors:
In all dc machines, the speed and torque are given by expressions-
Torque and Speed of a DC Motor:
Equations (1.26) and (1.27) reveal that-
The above relations reveal that increase in flux would cause increase in torque and decrease in speed. It cannot be so because torque is the cause of rotation and increase in torque should cause increase in speed rather than decrease.
The apparent inconsistency between the above two relations may be reconciled as-
Let the motor be running at a steady speed and the field strength be weakened, which is possible by inserting an additional resistance in the field circuit in case of a dc shunt motor connected across constant voltage supply. The immediate effect is that, owing to the momentary reduction in back emf, the current flowing through armature circuit increases out of all proportion to the reduction in flux.
As a result, inspite of the weakened field, the torque increases momentarily tremendously and will exceed considerably the value corresponding to load. The surplus torque, thus available causes the motor to accelerate, and the back emf to increase. The motor will attain steady speed again when the back emf has attained such a value that the current flowing in the armature develops with the slight weakened flux, only just sufficient torque to drive the load.
For avoiding undue rush of current the field strength should be reduced slowly.
The manner in which the motor slows down on strengthening of the field is also of equally importance. When the field is strengthened, the immediate effect is increase in back emf. Quite a small increase in the flux will normally be sufficient to increase the back emf beyond supply voltage, and in consequence, will not merely reduce the value of armature current, but will cause it to reverse its direction of flow.
The machine will, therefore, act temporarily as a generator, and feed energy to the supply system, this energy being available at the expense of the kinetic energy associated with the machine and its load, with the result that a rapid reduction in speed takes place. The decrease in speed will cease when the back emf has fallen to a value such that the motor takes just sufficient current to develop the torque the load requires. By thus strengthening the field the motor speed is reduced by a form of “regenerative braking”.
Now it is obvious that torque developed in a motor is certainly a function of flux and armature current and is independent of speed and the speed depends upon the torque.
(b) Starting Characteristics of Electric Motors:
The study of starting characteristics of a motor is essential to know whether the starting torque that the motor is capable of developing is sufficient to start and accelerate the motor and its load to the rated speed in a reasonable time or not. This feature is very important particularly when the motor is to be selected for the services in which either it is to start against full-load torque (as for driving grinding mills or expellers) or it is to be stopped and started very frequently (as in case of lifts and hoists), i.e., where high starting torque is essential.
The torque developed by the motor at the starting is required, (i) to overcome the initial static friction and (ii) accelerate the motor and load to the full speed. The torque required to overcome the initial static friction cannot be easily determined and in certain cases as for starting a machine, which has been out of use for some time, it may be much more than the full-load torque.
The torque for accelerating depends upon the load torque.
The loads which are usually met with may be divided according to accelerating torque requirements into the following categories:
(i) Load requiring very small accelerating torque in comparison with full-load torque such as when the motor is to be run light.
(ii) Load requiring the torque which may increase with speed and it may be proportional to (speed)2 as in case of centrifugal pumps or fans.
(iii) Load having constant load torque at all speeds as in case of hoists, lifts etc.
(iv) When the motors have to start and accelerate against full-load torque and in addition to accelerate some heavy moving parts as in case of rolling mills or with motors equipped with a flywheel.
It is obvious that starting of a motor in category (i) is quite a different proposition from starting in category (iv), even though the full-load torque and speed may be the same in each case; a study of the load characteristic is thus necessary in order to be able to choose the most suitable type of motor and starter.
Another factor of importance is the time taken to run the motor up to speed, since this governs the heating of the motor and control equipment because of the heavy starting currents.
(c) Speed Control:
Speed control means intentional change of the drive speed to a value required for performing the specific work process. This concept of speed control or adjustment should not be taken to include the natural change in speed which occurs due to change in the load on the drive shaft. The desired change in speed is accomplished by acting accordingly on the drive motor or on the transmission connecting it to the unit it serves to drive. This may be done manually by the operator or by means of some automatic control device.
Any given piece of industrial equipment may have its speed changed or adjusted mechanically by means of stepped pulleys, sets of change gears, variable speed friction clutch mechanism, and other mechanical devices. Historically, this proved to be the first step in transition from non-adjustable speed to adjustable speed drive. The electrical speed control has many economical as well as engineering advantages over mechanical speed control.
The nature of the speed control requirement for an industrial drive depends upon its type. Some drives may require continuous variation of speed for the whole of the range from zero to full speed, or over a portion of this range; while the others may require two or more fixed speeds. Some machines may require creeping speed for adjusting or setting up the work. Such a speed is of the order of few rpm. For most of the drives, however, a control of speed within the range of ± 20% may be suitable.
Some of the important terms related to speed are defined below:
i. Constant Torque Drive:
It is a drive in which the motor shaft torque remains constant over a given speed range. In such a drive, shaft power varies as the speed.
ii. Constant Power Drive:
It is a drive in which the motor shaft power, given by the product of shaft torque and speed, remains constant over a given speed range. In a constant power drive, higher torques are available at lower speeds and vice versa. The motor size is always determined by the highest torque requirement at the lowest speed.
Where, N0 and Nf are no-load and full-load speeds respectively.
(d) Electric Braking:
If the load is removed from an electric motor and supply to it be disconnected, it will continue to run for some time due to inertia. The time elapsing before it stops will be especially long if the motor is massive and has run at high speed. It is essential, however, in many cases that the motor and its driven machine be stopped quickly (in machine tools, cranes, hoists etc). In fact, quick stopping of a motor is more essential than quick starting. Delay in starting up a motor only causes the machinery to stand idle; a delay in stopping a motor may result in heavy damage to equipment or to the manufactured products and even the loss of human life.
Based on the purpose for which braking is employed, it is of the two types, viz, braking while bringing the drive to rest and braking while lowering loads. In the first type, the device employed for braking absorbs the kinetic energy of the moving parts while in the second one, it absorbs, in addition to the kinetic energy, potential energy, usually gravitational which can drive the system at an excessively high speed.
Braking, while stopping, may be used for any one of the following objectives:
(i) Reducing the time taken to stop.
(ii) Stopping exactly at specified points, for example in lifts; sometimes such precise stops are necessary for reasons of safety.
(iii) Feeding back, atleast a portion of the power, to the supply system.
Braking, while lowering loads, may be employed for any of the following purposes:
(i) Controlling the speed at which the load comes down and limiting it to a safe value.
(ii) Feeding power back to the supply system.
The main features of a satisfactory brake are that:
(i) The braking should be quick and reliable in action,
(ii) The braking torque must be controllable,
(iii) Some suitable means must be provided for dissipation of kinetic energy of the moving parts of the motor and its driven machines and
(iv) Failure of any part of the braking system must result in application of brakes.
Braking torque can be applied either by mechanical (or friction) brakes or electrodynamically. In mechanical braking, the stored energy of rotating parts is dissipated in the form of heat by a brake shoe or band rubbing on a wheel or a brake drum. In electrical (or electrodynamic) braking the stored energy of rotating parts is converted into electrical energy and dissipated in the resistance in the form of heat or returned to the supply. Here we will deal with electric (or electrodynamic) braking only.
Let electrical and mechanical torque in N-m be TE and TF respectively.
Electrical torque,
Te ∝ speed of rotation if excitation is constant.
Braking torque,
TB = TE + TF = Kω + TF
Angular retardation,
Time required to bring the motor from an initial speed ω1 final speed ω in the same direction is given as
Time required to bring the motor to standstill is obtained by substituting ω = 0 in above equation,
Number of revolutions made,
5. Mechanical Considerations:
(a) Types of Enclosures:
In order to protect machines against the ingress of dirt and dust or larger foreign bodies like spanners, vermin etc. into it, it is desirable that some suitable enclosure be provided. But the enclosure provided for this protection will interfere with the free entry of cool air into the machine with the result that an enclosed machine does not give the same output as does an open type machine of the same dimensions for a given temperature rise.
The purpose of providing enclosure is twofold—firstly providing protection to persons against contact with live or moving parts inside the enclosure and protection to machines against the ingress of solid foreign bodies, and secondly providing protection to machines against harmful ingress of water. Various types of enclosures giving various degrees of protection and cooling facilities for use in different working conditions can be built. ISI 4691 provides an ingress protection code which consists of the letter IP followed by two numbers—the first characteristic numeral designates the extent of protection of persons against contact and of the machine against ingress of solid foreign bodies, while the second characteristic numeral indicates the extent of protection to the machine against harmful ingress of water.
Further addition of letter S or M indicates that the degree of protection against ingress of water is applicable only while the machine is stationary or rotating respectively. Absence of both the letters means that protection is provided under both the conditions. For example enclosure marked IP 22 will provide protection against ingress bodies of diameter more than 12 mm and against ingress of water falling up to 15° from vertical under both the conditions.
Similarly enclosures marked IP 44 and IP 56 will provide protection against ingress of bodies of diameter more than 1 mm and against water splashed under both the conditions and protection against ingress of bodies in powder form and water projected by a nozzle under both the conditions respectively.
(b) Types of Bearings:
Bearings are those parts of a machine which house supports and restrain the rotating parts. The moving part of the bearing is known as journal and the stationary part that supports the moving part is known as bearing.
The main functions of a bearing, besides supporting the rotating parts, are:
(i) To permit free rotation of the moving components with minimum friction and
(ii) To maintain the rotating member of the machine in a fixed physical location relative to the stationary member.
The bearings are of two types, namely:
1. Ball or roller bearings and
2. Sleeve or bush bearings.
1. Ball or Roller Bearings:
Ball or roller bearings are those bearings in which shafts are supported by balls or rollers which themselves are movable. The action in ball or roller bearings is not that of rubbing but of rolling between the balls and or rollers and their races, the inner surface being fixed to the shaft while the outer surface is fixed to the housing supporting the bearing. Such bearings are supplied and filled with grease. The period between the renewals of grease depends upon the speed of rotation of the motor. These are also known as antifriction bearings.
Up to about 75 kW use of ball or roller bearings is made. Very small motors are usually provided with ball bearings. In vertical shaft drives the ball bearings are essential as they are capable of taking an axial thrust.
Though the initial cost of bearings of this type is high but their life is longer, occupy less space, friction loss is smaller and their maintenance cost is low. These have made use of induction motors with small air gap possible as the roller bearing does not wear appreciably. However, these cannot be recommended where noise is to be avoided.
2. Sleeve or Bush Bearings:
Sleeve or bush bearings are those bearings in which the rotating shaft is supported by a bearing component rigidly fixed to the frame of the machine.
They may be:
(i) Journal bearing offering support to the shafts at right angles to the shaft axes,
(ii) Foot-step bearings supporting shafts parallel to the shaft axes and
(iii) Thrust or collar bearings offering support to shafts subjected to end or axial thrust.
Sleeve bearings are normally of bronze but for universal and fan-cooled motors, sintered powder or porous metal bearings may be used because of their self-lubricating properties due to capillary action.
Sleeve bearings on very small motors are lubricated with an oil wick pressed lightly against the shaft by a spring. The wick must be kept saturated with fairly light oil and will remain moist through several weeks of heavy use. Large sleeve bearings are ring lubricated which consists of a ring freely rotating on the shaft, carrying oil to the bearings. Lubricating oil, which is free from acid and resin, is renewed every six months. Temperature of bearing (either sleeve or roller) is the indication of the bearing condition. ISS 325 provides for maximum temperature rise at sleeve or ball bearings to be 50°C.
Bearings of this type are necessary to reduce the noise level for certain types of couplings and for the positions subject to external vibrations. They are also used where there is a danger of individual balls or rollers indenting the race during long rest periods.
Motors with sleeve bearings should always be used with shaft horizontal and fixed coupling between motor and the driven machine. Because of larger wear of the bearings, relatively large air gap in the induction motor has to be introduced.
(c) Types of Transmission of Drive:
Mechanical power available at the motor shaft has to be transmitted to the driven machine or machines. There are various methods of doing so.
Among these may be mentioned the following:
1. Direct Drive:
In this type of drive the driving member is connected direct to the driven member, without any interposed gearing, by means of solid or flexible coupling. It is the simplest method, space required is less than with belt drive and efficiency is 100 per cent. Solid coupling requires very accurate aligning otherwise there is possibility of damage to shafts. Another shortcoming of solid coupling is that sudden jerks of load are transmitted to the motor. By employing flexible coupling both of the above shortcomings are overcome to some extent. Flexible coupling can accommodate some angular, lateral or vertical misalignment.
The other disadvantages of direct drive are that:
(i) End thrust is exerted on the motor by the load if it happens to be a centrifugal pump and
(ii) The driving and driven members must run at the same speed, which is usually not desirable.
This type of drive is employed where there is a possibility of arranging the driving member in line with the driven member and where speed of driven member is same as that of driving member.
2. Belt Drives:
These are the least expensive drives. These are employed where a speed change is desired in the transmission of power and where it is not absolutely essential to maintain a fixed speed ratio between the driving and driven shaft.
The belt drives are of two types namely:
(i) Flat belt drive and
(ii) V-belt drive.
3. Rope Drive:
Rope drive, the drive by means of ropes running over pulleys having a number of grooves, is used for power ranges beyond the limit of V-belt drive. It is a long centre drive. Main advantages of rope drive are negligible slip and ability of taking sudden loads. Where the speed ratio is very high rope drive is indispensable. This type of drive is coming into use on small and medium sized machines, nowadays.
Coupled motors driving through multiple ropes working in grooved pulleys is shown in Fig. 1.113 (c).
4. Chain Drive:
Though chain drive is more costly than belt and rope drives but it is more efficient, can be used for high speed ratio (the limit being 7:1 with a distance between pulley centres of about 1.5 to 2 times the diameter of the larger pulley) and has no slip. It can be used conveniently for drives involving up to 12 or more parallel shafts; the driving centres can be shorter than with any drive except the direct or gear drive. It is particularly suitable for damp and dirty conditions because the chain case protects the drive from outside influences.
With chain drive, it must be ensured that shafts are parallel and chains run centrally on their sprockets at right angle to the shafts to give long life and avoid end thrust on the motor. This method is suitable for speeds up to 900 m/minute needing positive drives without any slipping. Chain drive, through reduction gearing, from a 2.5 kW motor is shown in Fig. 1.113 (d).
5. Gear Drive:
It is a short centre positive drive. It is very important to have proper alignment in the absence of which, otherwise, we may have to pay in the form of bent motor shafts. It was the first form of power transmission in the early steam driven mills. Modern straight cut gears, and the particular variation found in the worm reduction drive, are common features in many heavy machines such as cloth calendars. A typical 35 kW worm gear with a speed reduction of 15 : 1 will have a mechanical efficiency of about 98 per cent.
6. Vertical Drive:
In this type of drive motor is arranged with its axis vertical. This arrangement frequently proves to be convenient.
Choice of Drive:
Choice of drive is governed by the speed of driving and driven machines, convenience, space available, clutching arrangement required and cost. Choice of speed of motor is most important factor as it not only affects the performance of the motor but also overall cost. The dimensions and, therefore, the first cost of a motor for a given output are approximately inversely proportional to the speed, so for the same output kW the cost of high-speed motor is less than that of a low-speed motor. The efficiency and, in case of induction motors, power factor also decreases with the decrease in speed.
Thus for a low-speed drive high-speed motor using a reduction gear is usually found cheaper than a low-speed direct coupled motor. These days there is a trend to make motors for low-speed drives by incorporating gear in the motor frame, the motor being designed for a high speed. The overall cost of such a motor including the gear is less than that of the corresponding low-speed motor; gear efficiency ranging from about 96 per cent; for a 5 : 1 ratio to about 80 per cent for a 50 : 1 ratio can be obtained.
(d) Noise Level:
A feature of importance in choosing a motor, as well as any other apparatus, is that of noise; extreme silence is, of course, essential in motors for domestic purposes, use in hospitals, theatres, another similar institutions. Even for ordinary industrial purposes it is essential to keep motor noise to a minimum because it causes fatigue to the workers and, therefore, affects the overall efficiency and output of the factory adversely. The increasing use of steel-framed buildings, which transmit noise readily, tends to make the problem of greater importance than hitherto.
The level of sound is measured in bels (B), which is defined as:
Difference in loudness:
Where, W1 and W2 are the powers of sound waves under consideration and P1 and P2 are the pressures corresponding to the sound waves concerned.
In practice the decibel (one-tenth of a bel) is more convenient unit and represents approximately the smallest difference in sound which the ear can detect.
Thus the difference in loudness between two sounds-
Noise may be due to purely mechanical causes, such as resonance or vibration of laminations, or it may be due to magnetic pulsations caused by an incorrect choice of the number of slots. The elimination of such features is a matter of the manufacturer, but their effects can be minimized by preventing transmission of vibrations to other parts of the building. This can be accomplished by resilient or spring mounting the motor.
Resilient mounting makes use of cushioning material such as felt mat, cork and rubber slabs which is effective in the audible frequency range. Spring mounting on the other hand can absorb all vibration frequencies. To avoid transmission of vibrations electrical connections to the motor are made by means of flexible pipe rather than by conduit pipe. It is, however, impossible to exterminate the noise.
(e) Insulating Materials:
Insulating materials are employed for providing electrical insulation between parts at different potentials. These materials begin to deteriorate at relatively small temperatures. For reliable operation, it is essential that the temperature rise in electrical machines and equipments do not exceed the permissible maximum temperature of the insulating materials used therein.
The three fundamental electrical properties of insulating materials of utmost importance in the operation of electrical machines or equipment are resistivity (insulation resistance), dielectric strength and dielectric loss angle. In addition to the above, other important properties of insulating materials are mechanical strength, heat resistance, hygroscopicity.
An insulating material should have high resistivity, high dielectric strength, low dielectric loss, good heat conductivity, sufficient mechanical strength to withstand vibrations etc., and capability of withstanding a repeated heat cycle without deterioration and should be non-hygroscopic.
The materials used for insulation in electrical machines and apparatus are numerous (such as insulating varnishes, mineral waxes, synthetic waxes, varnish, cloth, silk, cotton, rubber, vulcanized rubber, hard rubber, bakelite, paper, wood, mica, asbestos, glass, porcelain, marble, slate, insulating oils etc.).
Temperature Rise in Electrical Machines:
Heat is developed in all electrical machines due to the losses in the various parts, principally copper loss (I2R loss) in conductors, eddy current and hysteresis loss in iron and mechanical losses (in rotating machines only) due to friction of the bearing, air friction or wind-age causing the temperature of that part to rise. This temperature rise continues until all the heat generated is dissipated to the surroundings by one or more of the natural modes of heat transfer, viz. conduction, convection and radiation.
Ultimately then, under steady load, each part achieves a final temperature, the magnitude of which depends on the balance between the rate at which heat is developed in that part (or received by conduction from a hotter part), and the rate at which the heat can be dissipated which is determined by effectiveness of the cooling method.
The temperature rise depends upon:
(i) The amount of heat produced and
(ii) The amount of heat dissipated per 1°C rise of the surface of a machine.
According to Newton’s law of cooling the rate of loss of energy of a hot body is proportional to the difference in temperature between that body and its surroundings. This law is approximately true for moderate temperature difference (up to 100° C) and for bodies dissipating heat by radiation and natural convection. It means that the amount of heat dissipated per 1° C rise of the surface of a machine depends on the surface area of cooling.
The size of motor for any service is governed by the maximum temperature rise when operating under the given load conditions and the maximum torque required. The former is more important because if the motor operates satisfactorily for maximum temperature rise, it will usually provide the required maximum torque, except in special cases where the load consists of heavy peaks followed by relatively long intervals of no load. Electrical machines are, therefore, designed for a limited temperature rise.
In fact, the continuous rating of a machine is that rating for which the final temperature rise is equal to or just below the permissible value of temperature rise for the insulating material used in protection of motor windings. When the machine is overloaded for such a long time that its final temperature rise exceeds the permissible limit, it is likely to be damaged. In worst cases, it will result in an immediate thermal breakdown of the insulating material which will cause a short circuit in the motor, thus putting a stop to its functioning.
The short circuit may also lead to a fire. In less severe cases immediate thermal breakdown of the insulating material may not occur, but the quality of the insulation will deteriorate such that thermal breakdown with future overloads or even normal loads might soon occur, thus shortening the useful life of the machine.
The temperature rise to which a motor be allowed to rise is limited by the insulation. The maximum temperature rises which should not be exceeded by different types of motor are fully set out in the relevant ISS.
Since temperature rise is one of the chief features in fixing the size of a motor, its calculations become a matter of importance and may be carried out as detailed below:
i. Heating-Time Curve:
For determination of an expression for the temperature rise of an electrical machine after time t seconds from the instant of switching it on, let-
Power converted into heat = P joules/s or watts
Mass of active parts of machine = m kg
Specific heat of material = Cp joules/kg/°C
Surface area of cooling = S metres2
Coefficient of cooling = α in watts per metre2 of surface per °C of difference between surface and ambient cooling temperatures.
ii. Assumptions Made:
(a) Losses or heat produced remains constant during the temperature rise.
(b) Temperature of the cooling medium remains unchanged, and
(c) Heat dissipated is directly proportional to the difference in temperature of the motor and the cooling medium.
Suppose a machine attains a temperature rise of θ°C above ambient temperature after t seconds of switching on the machine and further rise of temperature by dθ in very small time dt.
Energy converted into heat = Pdt joules
Heat absorbed = mCpdθ joules
Heat dissipated = Sθαdt joules
Since energy converted into heat
= Heat absorbed + heat dissipated
When final temperature is reached, there is no absorption of heat. Whatever heat is generated, has to be dissipated.
Integrating both sides, we get-
Where K1 is a constant of integration.
Substituting t = 0, θ = θ1, initial temperature rise from initial conditions, we get-
Where, τ = mCp/Sα and is known as heating time constant.
If motor is started from ambient temperature, θ1 = 0, we have-
Heating time constant is defined as the time taken by the motor in attaining the final steady value if the initial rate of rise of temperature were maintained throughout the operation. Substituting t = τ in Eq. (1.97), we have-
θ = θF (1 – e-1) = 0.632 θF … (1.98)
So heating time constant may also be defined as the time duration during which the machine will attain 63.2% of its final temperature rise above ambient temperature.
From Eq. (1.94) it is obvious that θF is directly proportional to the power losses and inversely proportional to the surface area S and specific heat dissipation α.
For poorly ventilated machines, it will attain a higher final temperature rise.
Heating time constant τ, being equal to mCp/Sα, has small value for well ventilated machines and large value for poorly ventilated machines. Large size machines have large heating time constant because with the increase in size of machine, the volume and hence the mass increases in proportion to the third power of linear dimensions and surface area S in proportion to second power. Typical values of heating time constant lie between about 1½ hours for small motors (7.5 kW to 15 kW) up to about 5 hours for motors of several hundred kW.
iii. Cooling-Time Curve:
Let the machine be switched off after reaching final steady-temperature rise of θF. When the machine is switched off, no heat is produced, therefore,
0 = Heat absorbed + heat dissipated or
0 = mCpdθ + Sθα’dt where α’ is the rate of heat dissipation during cooling
Integrating both sides, we have-
Where K2 is a constant of integration.
Substituting t = 0, θ = θF from initial condition in above equation, we have-
Substituting K2 = mCp/Sα’ loge θF in Eq. (i), we have-
Where, τ’ = mCp/Sα’, called the cooling time constant.
Substituting t = τ’ in Eq. (1.99), we have-
i.e., cooling time constant may also be defined as the time required to cool the machine down to 0.368 times the initial temperature rise above ambient temperature.
The heating and cooling curves follow an exponential law. Heating time constant and cooling time constant may be different for the same machine, as the ventilation conditions in the two cases may not be the same. The cooling time constant of a rotating machine is usually larger than its heating time constant owing to poor ventilation conditions when the machine cools. In self-cooled rotating machines the cooling time constant is about 2-3 times greater than its heating time constant because cooling conditions are worse at standstill.
6. Service Capacity and Rating:
(a) Duty Cycles:
The nominal duty of a drive motor is the duty corresponding to the service conditions and performance marked on its name plate.
There are three types of duties viz., continuous duty, short- time duty and intermittent duty.
The heating and cooling curves for continuous-duty motor are shown in Fig. 1.115 (a). Continuous duty is that duty when the on-period is so long that the motor attains a steady-state temperature rise.
The heating and cooling curves for short-time duty motor are given in Fig. 1.115 (b). The short-time duty motor operates at a constant load for some specified periods which is then followed by a period of rest. The period of run (or load) is so short that machine cannot attain its steady temperature rise while the period of rest is too long that the motor temperature drops to the ambient temperature.
The heating and cooling curves for intermittent periodic duty motors are illustrated in Fig. 1.115 (c). On intermittent duty the periods of constant load and rest with machine de-energized alternate. The loading periods are too short to allow the motor to attain its final steady-state value while periods of rest are too small to allow the motor to cool down to the ambient temperature. Intermittent rating of a machine is defined as the load which is applied during a certain fraction of time of a load cycle and the temperature rise limit is not exceeded.
When a machine is intermittently loaded, it will cool down during the time it is off, and temperature will rise when it is on, as illustrated in Fig. 1.116.
Continuous duty motors are employed to drive fans, compressors, generators etc. They may be in operation for many hours and even days in succession. Short-time duty motors are used in navigation-lock gates, railway turntables, bascule bridges, and the like. Intermittent-duty motors are employed in cranes, hoists, lifts, rolling mills, some metal- working machines, etc.
The duty cycles for various motor duties are shown in Fig. 1.117.
Maximum Temperature Attained With Intermittent Loading:
When a machine is intermittently loaded it will cool down during the time duration it is off and its temperature will rise when it is on, as illustrated in Fig. 1.116. Let θ1, θ’1, θ”1, be θ'”1 be the temperature rise after heating and θ2, θ’2, θ”2, θ'”3 be the temperature rise after cooling, as illustrated in Fig. 1.116.
For n times intermittency, we have-
When n = ∞ both enx and eny will be zero as x and y are negative. If θF is the maximum temperature rise with intermittent load then-
Rating of Machines:
A name plate fixed to the outside frame of an electrical machine records the data pertaining to its rating. A machine rating specifies the voltage, current, speed, excitation, power factor, efficiency, power output etc.
The rating of a machine should give all information necessary so that, if the machine is operated within the limits of all factors specified in its rating, the machine will operate satisfactorily and safely and will give reasonable length of service. Therefore, the rating of a machine must give the necessary information to safeguard the application of the machine from conditions of operation which (i) would result in unsafe mechanical or electrical strains upon any part of its structure or (ii) would result in excessive deterioration of the mechanical or electrical characteristics of the materials of which the machine is constructed. To give this information, the rating of an electric machine should include the output, voltage, speed, and any other information that may necessary for the proper operation of the machine.
Generators and motors are rated in terms of kW output at a given speed and voltage. The size and rating of an electric machine for any service is mainly governed by the factor ‘temperature rise’. The maximum temperature, to which an electric machine is allowed to reach, is limited by the type of insulation used.
The maximum temperature rise permissible with insulation A and insulation B are 40°C and 50°C respectively. Overloads are generally permissible for short period of time but when machines are required to carry greater loads than those specified, they must be kept under inspection to see that the temperature does not rise too much and that severe sparking at the commutator does not occur.
The type of service to which a machine is subjected is of great importance. The machines operating continuously at rated (or near rated) load are physically larger than those working at intermittent loads. Also electric machines that are not enclosed and are, in addition, well cooled by fans are likely to have higher ratings than covered up machines or those machines located where air does not circulate freely through and over them. High-speed machines and machines employing mica, glass tapes, and the new silicon as insulation can generally be physically smaller, in given ratings, than the low-speed machines employing standard insulations. The output of dc machines is also limited by the factor ‘commutation sparking’. This factor often limits the output, even though heating may not have proceeded to permissible values.
The machines according to ISI specifications are classified as follows:
i. Continuous Rating:
This is an output, which a machine delivers continuously without exceeding the permissible temperature rise. It can deliver 25 per cent overload for two hours.
ii. Continuous Maximum Rating:
Similar to continuous rating but not allowing any overload. It is used for motors of capacity larger than 1.84 kW (2.5 hp) per rpm.
iii. Short-Time Rating:
This is an output which an electric machine can deliver for a specified period (say 1 hr., 1/2 hr., 1/4 hr. etc.) without exceeding the specified temperature rise.
Effect of Altitude on Rated Output:
At high altitudes, the density of cooling air decreases. As a result, cooling capacity is reduced. The effect is negligible for elevations not exceeding 1,000 m.
The output is reduced as stated below in tabular form:
Overload Capacity of Induction Motors:
Motors are designed to carry overloads, without any adverse effect, as mentioned below:
Single phase induction motors are usually not designed to carry overload. However, 20-25% overload in torque intermittently may be permitted in split-phase ac motors and 40-45% overload in torque intermittently for capacitor-start motors.
Choice of Rating of Motors:
The choice of ratings of motors for the services requiring fairly constant power such a fans, air-compressors, blowers, pumps, motor-generator sets etc. is every simple. In such cases the power required is determined and motor having continuous rating of power required is chosen.
It is, however, worthwhile to take considerable trouble to find out exactly how much power is actually required by the driven machinery, since if the motor chosen be too small it will be overloaded and will, therefore, overheat and deteriorate, while if it is too large the efficiency and power factor will probably be less than those of a motor of the correct size. If there is no possibility of overloading then a motor with a continuous maximum rating may be selected, which will be comparatively smaller in size and cheaper in cost. Such a motor may operate at a higher temperature (55°C in place of 40°C).
For the constant torque loads, the size of the motor can be determined as follows:
In case of rotary motion, rating of motor required is given as-
Where, T is the load torque in N-m, N is the speed in rpm and η is the product of the efficiency of the driven load and that of the transmitting device. Efficiency varies considerably with the type of drive, bearings etc. and its value can best be dictated by experience in applications.
In case of linear motion, rating of the motor required is given as-
Where, F is the force caused by the load in kg and v is the velocity of motion of load in m/s.
The above expression [Eq. (1.103)] can be applied in case of hoisting mechanism, lifts or elevators with slight modification and that is due to counterweight (balancing the weight of the cage or car as well as one-half of the useful load). The modified expression is-
v in case of normal passenger lift cabins lies in the range of 0.5-1.5 m/s.
In case of pumps, the rating of the motor required,
where Q is the delivery of pumps in m3/s, ρ is the density of liquid pumped in kg/m3 and h is the gross head (static head + friction head) in m. Efficiency η lies in the range of 80 to 90% in case of reciprocating pumps and 40 to 80% in case of centrifugal pumps.
Similarly, the rating of a fan motor is given as-
Where, Q is the volume of air or any other gas in m3/s and h is the pressure in mm of water or in kg/m2. Efficiency may be taken to be 0.8 for larger power fans and 0.6 in case of smaller power fans.
The selection of size of motor is also simple for the services where the motor is run for a short time so that the temperature rise is less than the maximum permissible value and then it is allowed to cool down to the ambient temperature. For such services a motor rated at the required output power for the given period is chosen.
In cases where the load fluctuates over a given cycle, as in rolling mills or colliery winders, the size of the motor is determined accurately by finding the heating and cooling curves of motors under consideration and plotting heating curve for each motor, when working on the given cycle. The smallest motor which delivers the load according to given cycle without exceeding the specified temperature rise may be chosen.
Other methods for determination of rating of a motor for continuous duty and variable load are given below:
1. Method of Average Losses:
In this method it is assumed that the temperature rise attained by the motor with fluctuating loading conditions over a certain period of time will be the same as that attained by the motor with a certain load of constant magnitude. This will be true provided that the average losses in the motor for the same time are the same for both the conditions of operation.
This method consists of determining average losses in the motor when it operates according to the given load diagram and then comparing with the losses corresponding to the continuous duty of the machine when operated at its nominal rating.
2. Equivalent Current Method:
This method is based upon the assumption that the actual variable current may be replaced by an equivalent current Ieq which produces the same losses in the motor as the actual current.
The heating and cooling conditions in self-ventilated machines depend upon its speed. At low speeds the cooling conditions are poorer than at normal speeds. Therefore, if the work cycle involves slow-speed operation, it must be taken into consideration when using Eq. (1.107).
The equivalent current is compared with the rated current of the motor selected. The equivalent current may be less than or equal to the rated current of the machine.
3. Equivalent Torque Method:
This method is based upon the assumption that the motor current is proportional to the torque and heating is proportional to the square of current (i.e., heating is proportional to the square of torque).
The equivalent torque is determined in the same manner as the equivalent current i.e.,
A motor having torque nearly equal to Teq is selected.
4. Equivalent Power Method:
At constant speed or where the variations in speed are small, the equivalent power is given by the relationship,
A motor having power rating Peq is selected.
The equivalent current method is the most accurate out of the four methods discussed above. This method may be employed for determining the capacity for all applications except where it is necessary to take into account the variations in so called ‘constant losses’.
The equivalent torque method cannot be employed for applications where equivalent current method cannot be applied or in cases in which flux does not remain constant like dc series motors.
The equivalent power method cannot be used for motors whose speed varies considerably under load, especially when dealing with starting and braking conditions.
A name plate fixed to the outside frame of an electrical machine records the data pertaining to its rating. A machine rating specifies the voltage, current, speed, excitation, power factor, efficiency, power output etc.
The rating of a machine should give all information necessary so that, if the machine is operated within the limits of all factors specified in its rating, the machine will operate satisfactorily and safely and will give reasonable length of service. Therefore, the rating of a machine must give the necessary information to safeguard the application of the machine from conditions of operation which (i) would result in unsafe mechanical or electrical strains upon any part of its structure or (ii) would result in excessive deterioration of the mechanical or electrical characteristics of the materials of which the machine is constructed. To give this information, the rating of an electric machine should include the output, voltage, speed, and any other information that may necessary for the proper operation of the machine.
Generators and motors are rated in terms of kW output at a given speed and voltage. The size and rating of an electric machine for any service is mainly governed by the factor ‘temperature rise’. The maximum temperature, to which an electric machine is allowed to reach, is limited by the type of insulation used.
The maximum temperature rise permissible with insulation A and insulation B are 40°C and 50°C respectively. Overloads are generally permissible for short period of time but when machines are required to carry greater loads than those specified, they must be kept under inspection to see that the temperature does not rise too much and that severe sparking at the commutator does not occur.
The type of service to which a machine is subjected is of great importance. The machines operating continuously at rated (or near rated) load are physically larger than those working at intermittent loads. Also electric machines that are not enclosed and are, in addition, well cooled by fans are likely to have higher ratings than covered up machines or those machines located where air does not circulate freely through and over them. High-speed machines and machines employing mica, glass tapes, and the new silicon as insulation can generally be physically smaller, in given ratings, than the low-speed machines employing standard insulations. The output of dc machines is also limited by the factor ‘commutation sparking’. This factor often limits the output, even though heating may not have proceeded to permissible values.
The machines according to ISI specifications are classified as follows:
i. Continuous Rating:
This is an output, which a machine delivers continuously without exceeding the permissible temperature rise. It can deliver 25 per cent overload for two hours.
ii. Continuous Maximum Rating:
Similar to continuous rating but not allowing any overload. It is used for motors of capacity larger than 1.84 kW (2.5 hp) per rpm.
iii. Short-Time Rating:
This is an output which an electric machine can deliver for a specified period (say 1 hr., 1/2 hr., 1/4 hr. etc.) without exceeding the specified temperature rise.
Effect of Altitude on Rated Output:
At high altitudes, the density of cooling air decreases. As a result, cooling capacity is reduced. The effect is negligible for elevations not exceeding 1,000 m.
The output is reduced as stated below in tabular form:
Overload Capacity of Induction Motors:
Motors are designed to carry overloads, without any adverse effect, as mentioned below:
Single phase induction motors are usually not designed to carry overload. However, 20-25% overload in torque intermittently may be permitted in split-phase ac motors and 40-45% overload in torque intermittently for capacitor-start motors.
Frame Size:
A frame is the mechanical structure required to house a stator along with its bearings, end covers and terminal box. The standardization of frame size leads to economy. All modern motors of small size are built with standard frame sizes as specified in IEC 72 which lists a coherent range of main structural dimensions with centre heights between 56 and 1,000 mm.
A frame size is designated by a number which is its centre height (H) expressed in mm. Thus frame designated 132 has a centre height-(H) of 132 mm. The frame sizes with heights between 56 and 1,000 mm as per recommendations of IEC are 56, 63, 71, 80, 90, 100, 112, 132, 160, 180, 200, 225, 250, 280, 315, 355, 400, 450, 500, 630, 710, 800, 900, 1,000 mm.
The outputs from the standard frames are periodically assessed to take into account the latest technological advancements.
The frames may be die-cast or fabricated. Machines of ratings up to 50 kW have their frames die-cast in a strong silicon aluminium alloy and in some cases with the stator core cast in. The frames of larger machines are fabricated by welding steel plates. The advantage of fabrication is its adaptability to new designs and modifications.
Load Equalization:
In many industrial drives, such as in rolling mills, planning machines, electric hammers, presses, reciprocating pumps, the load fluctuates over a wide range in the space of a few seconds. It is desirable to smoothen out the fluctuations in load, otherwise during interval of peak load, it will draw a heavy current from the supply either producing large voltage drop in the distribution system or requiring cables and wires of heavy section. The process of smoothing out of fluctuations in the load is known as load equalization. In this process, energy is stored during the interval of light load and given out during the interval of peak load. Thus the power drawn from the supply mains remains almost constant.
The most common method of load equalization is by use of flywheel. During the light load period the flywheel accelerates and stores the excessive energy drawn from the supply and during peak load period the flywheel decelerates and supplies some of its stored energy to the load in addition to the energy supplied from the supply. Thus the load demand is reduced.
The motors used for such load should have drooping speed-torque characteristics, so that the speed may fall with the increase in load and enables the flywheel to give up its stored energy. For the load in which the motor have to run in the same direction and is not to be stopped and started frequently, flywheel may be mounted on the motor shaft. For a reversing drive, such as for colliery winder, the Ward Leonard control system is generally used for reversing and speed control, so flywheel can be mounted on the shaft of the motor-generator set. The load torque required and motor torque developed as well as speed variations with time are shown in Fig. 1.125.
Flywheel Calculations:
Let the load torque (assumed constant during the time for which the load is applied) be TL in newton-metres. Flywheel torque = TF N-m
No-load torque = T0 N-m (say, due to friction and windage etc.)
Motor torque at any instant = Tm N-m
Motor speed on no load = ω0 radians/s
Motor speed at any instant = ω radians/s
Motor slip, s = ω0 – ω
Moment of inertia of flywheel = J kg-m2
Time = t seconds.
Case 1. Load Increasing:
During this period the flywheel decelerates and gives up a part of stored energy in it. The torque required to be supplied by the motor, Tm = TL – TF.
Energy given out by flywheel when its speed is reduced from ω0 to ω-
But ω0 – ω = s and ω0 + ω/2 is the mean speed and may be taken equal to ω, since drop of speed is assumed not to exceed 10 per cent.
.’. Energy given out = Jωs
The power given out by flywheel
= Rate of energy given up
Flywheel torque,
Expression for the torque thus becomes-
Since for value of slip up to 10 per cent, the slip is proportional to the torque,
Case 2:
Load Decreasing:
While load decreases, the motor accelerates the flywheel to its normal speed, slip is therefore decreased and ds/dt is –ve.
Motor torque, Tm = T0 + TF
or Tm = T0 – J ds/dt since ds/dt is negative
By solving the above expression, we get-
Tm = T0 + (T’m – T0)e-t/KJ …(1.112)
Where, T’m is the motor torque at the instant when the load is removed.
7. Appearance:
Pleasing appearance is a good aspect of design, which can be obtained by harmonious blending of the motor finish, mounting and dimensions in line with those of the driven machine.
8. Cost:
Although the cost has been placed as last factor in the above list, but it is the major factor for the choice of motor. Sometimes we have to choose a motor which may not be an ideal choice from engineering consideration but cheap in cost—because special features are not worth the additional investment.
While considering the cost, the capital as well as running cost (cost of losses and cost due to maintenance, interest and depreciation) of a motor should be considered. In most of the cases, we find that equipment very cheap in first cost is costlier in running cost due to poor efficiency, low power factor and short life. In a nutshell the motor chosen should give the desired service at minimum overall cost.
As compared to plain squirrel cage 3-phase induction motors, which are the cheapest, the cost of dc motors range from 1.5 to 3 times its cost. The cost of synchronous motors, 3-phase slip-ring induction motors and commutator motors range from 1.5 to 4 times the cost of a plain squirrel cage induction motor of the same output. For single phase motors it is 1.5 to 2.5 times the cost of plain squirrel cage induction motor of the same size.